Layered photonic crystals

ABSTRACT

A three dimensional photonic crystal and layer-by-layer processes of fabricating the photonic crystal. A substrate is exposed to a plurality of first microspheres made of a first material, the first material being of a type that will bond to the templated substrate and form a self-passivated layer of first microspheres to produce a first layer. The first layer is exposed to a plurality of second microspheres made of a second material, the second material being of a type that will bond to the first layer and form a self-passivated layer of second microspheres. This layering of alternating first and second microspheres can be repeated as desired to build a three dimensional photonic crystal of desired geometry. Charged polymers such as polyelectrolyte coatings can be used to create the bonds.

CROSS REFERENCE TO RELATED DOCUMENTS

[0001] This application is a continuation in part of pending U.S. patentapplication Ser. No. 10/210,010, filed Jul. 31, 2002 entitled“Layer-By-Layer Assembly of Photonic Crystals” to John South Lewis, III,et al which is hereby incorporated by reference.

STATEMENT OF U.S. GOVERNMENT RIGHTS UNDER 35USC 202(c)(6)

[0002] The U.S. government has a paid-up license in this invention andthe right in limited circumstances to require the patent owner tolicense others on reasonable terms as provided for by the terms ofcontract number N66001-01-1-8938 awarded by the United States DefenseAdvanced Research Project Agency.

FIELD OF THE INVENTION

[0003] This invention relates generally to the field of photoniccrystals. More particularly, this invention relates to a photoniccrystal structure and a method for layer-by-layer fabrication of suchcrystal structure.

BACKGROUND

[0004] Photonic crystals are being actively pursued as components inoptical networks, such as wavelength-division multiplexing applications.Examples of potential applications are as filters, mirrors, waveguides,and prisms. Added functionality could allow the crystals to be used inother applications such as frequency-tunable filters, optical switches,chemical and biological recognition systems, as well as other potentialapplications.

[0005] Three dimensional (3-D) photonic crystals have been made using anumber of approaches. One common approach uses a colloidal techniquethat allows a distribution of spheres (e.g. microspheres—typicallyspheres approximately in the range of 90 nm to several microns indiameter) to settle out of solution into a bulk 3-D crystal. A similarapproach uses the surface tension of a moving liquid/gas interface,created by either by pulling a substrate out of a liquid or byevaporating the liquid, to create a 3-D crystal made of up microspheres.Both techniques result in a close-packed structure of identical spheres.More complex structures are possible if differently sized spheres areused, but there is very little external control over the crystallizationprocess and the resulting structure. The spheres can be made with anumber of different materials, with polystyrene a common example, andthe components are uniform in size and composition. Crystals fabricatedusing this technique are mechanically unstable unless a matrix such as apolymer matrix is used between the spheres to mechanically reinforce thestructure.

SUMMARY OF EXEMPLARY EMBODIMENTS OF THE INVENTION

[0006] The present invention relates generally to photonic crystals.Objects, advantages and features of the invention will become apparentto those skilled in the art upon consideration of the following detaileddescription of the invention.

[0007] In general terms, without any intention of limiting theinvention, the present invention, in certain embodiments, relates to atechnique which can be used to fabricate photonic crystals in acontrolled, layer-by-layer manner. This allows control over parametersnot possible with traditional colloidal techniques and permits novelcrystal structures to be created.

[0008] A method of fabricating a photonic crystal, consistent withcertain embodiments involves providing a substrate; exposing thesubstrate to a plurality of first microspheres made of a first material,the first material being of a type that will bond to the substrate andform a self-passivated layer of first microspheres to produce a firstlayer; and exposing the first layer to a plurality of secondmicrospheres made of a second material, the second material being of atype that will bond to the first layer and form a self-passivated secondlayer of second microspheres.

[0009] Another method of fabricating a photonic crystal, consistent withcertain embodiments involves a) providing a substrate; b) exposing thesubstrate to a plurality of first microspheres made of a first material,the first material being of a type that will bond to the substrate andform a self-passivated layer of first microspheres to produce a layer ofmicrospheres; c) modifying the first layer of microspheres to permit thefirst layer of microspheres to bond with other microspheres to therebyproduce a bondable layer; and d) exposing the bondable layer to aplurality of second microspheres to form a second layer of microspheres.

[0010] A photonic crystal structure, consistent with certain embodimentshas a substrate processed to bond preferentially to a first material inselected areas with a first layer of first microspheres, the first layerbeing one microsphere deep, the first microspheres comprising the firstmaterial and bonded to the selected areas of the substrate. A secondlayer of second microspheres one microsphere deep is bonded to the firstlayer of microspheres.

[0011] Another method of fabricating a photonic crystal, consistent withcertain embodiments involves providing a substrate; bonding a singlelayer of microspheres one microsphere deep to the substrate to form afirst layer; and bonding a single layer of microspheres one microspheredeep to the first layer to form a second layer.

[0012] Another method of fabricating a photonic crystal, consistent withcertain embodiments involves providing a templated substrate having afirst charge; and exposing the templated substrate to a plurality offirst microspheres having a polyelectrolyte coating carrying a secondcharge, the second charge being opposite the first charge so that theplurality of first microspheres will bond to the templated substrate andform a self-passivated layer of first microspheres to produce a firstlayer.

[0013] Another method of fabricating a photonic crystal, consistent withcertain embodiments involves: a) providing a templated substrate; b)exposing the templated substrate to a plurality of first microspheresmade of a first material, the first material being of a type that willbond to the templated substrate and form a self-passivated layer offirst microspheres to produce a layer of microspheres; c) modifying thefirst layer of microspheres to permit the first layer of microspheres tobond with other microspheres to thereby produce a bondable layer bycoating the first microspheres with a polyelectrolyte film having afirst charge; and d) exposing the bondable layer to a plurality ofsecond microspheres having charge opposite the first charge to form asecond layer of microspheres.

[0014] A photonic crystal structure, consistent with certain embodimentshas a templated substrate processed to bond preferentially to a firstmaterial in selected areas. A first layer of first microspheres, thefirst layer being one microsphere deep, (the first microspherescomprising the first material) is bonded to the selected areas of thetemplated substrate. A charged polymer coating is on the firstmicrospheres.

[0015] A method of fabricating a photonic crystal, consistent withcertain embodiments involves providing a templated substrate; bonding asingle layer of charged polymer coated microspheres one microsphere deepto the templated substrate to form a first layer; and bonding a singlelayer of charged polymer coated microspheres one microsphere deep to thefirst layer to form a second layer.

[0016] A method of fabricating a photonic crystal consistent withcertain embodiments involves bonding a single layer of charged polymercoated microspheres one microsphere deep to a substrate to form a firstlayer; and bonding a single layer of charged polymer coated microspheresone microsphere deep to the first layer to form a second layer.

[0017] A three dimensional photonic crystal and layer-by-layer processesof fabricating the photonic crystal consistent with certain embodimentshas a substrate that is exposed to a plurality of first microspheresmade of a first material, the first material being of a type that willbond to the templated substrate and form a self-passivated layer offirst microspheres to produce a first layer. The first layer is exposedto a plurality of second microspheres made of a second material, thesecond material being of a type that will bond to the first layer andform a self-passivated layer of second microspheres. This layering ofalternating first and second microspheres can be repeated as desired tobuild a three dimensional photonic crystal of desired geometry. Chargedpolymers such as polyelectrolyte coatings can be used to create thebonds.

[0018] The above summaries are intended to illustrate exemplaryembodiments of the invention, which will be best understood inconjunction with the detailed description to follow, and are notintended to limit the scope of the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The features of the invention believed to be novel are set forthwith particularity in the appended claims. The invention itself however,both as to organization and method of operation, together with objectsand advantages thereof, may be best understood by reference to thefollowing detailed description of the invention, which describes certainexemplary embodiments of the invention, taken in conjunction with theaccompanying drawings in which:

[0020]FIG. 1 illustrates an exemplary substrate consistent with certainembodiments of the present invention.

[0021]FIG. 2 illustrates a single layer of microspheres bonded to thesubstrate 100 in a manner consistent with certain embodiments of thepresent invention to create a single layer photonic crystal.

[0022]FIG. 3 illustrates a two layer photonic crystalline structure ofmicrospheres fabricated on the substrate 100 in a manner consistent withcertain embodiments of the present invention.

[0023]FIG. 4 illustrates a three layer photonic crystalline structure ofmicrospheres fabricated on the substrate 100 in a manner consistent withcertain embodiments of the present invention.

[0024]FIG. 5 is a perspective view of a substrate having etched invertedpyramids in an upper surface thereof consistent with certain embodimentsof the present invention.

[0025]FIG. 6 is a side cutaway view of an etched inverted pyramid alonglines A-A of FIG. 5 consistent with certain embodiments of the presentinvention.

[0026]FIG. 7 is a flow chart depicting a process for layer-by-layerfabrication of a photonic crystalline structure consistent with certainembodiments of the present invention.

[0027]FIG. 8 is a flow chart depicting an alternative process forlayer-by-layer fabrication of a photonic crystalline structureconsistent with certain embodiments of the present invention.

DETAILED DESCRIPTION

[0028] While this invention is susceptible of embodiment in manydifferent forms, there is shown in the drawings and will herein bedescribed in detail specific embodiments, with the understanding thatthe present disclosure is to be considered as an example of theprinciples of the invention and not intended to limit the invention tothe specific embodiments shown and described. In the description below,like reference numerals are used to describe the same, similar orcorresponding parts in the several views of the drawings.

[0029] The current invention, in its many embodiments, offers severalapproaches that could be used to build a photonic crystalline structurein a layer-by-layer fashion. This has the potential of improving thequality of the resultant crystals, but also offers the ability toengineer the structure of the crystal using spheres with varyingproperties, and substrates patterned to achieve varying effects.

[0030] One method, consistent with certain embodiments of the presentinvention, uses a type of biological recognition as a method forfabricating multi-layer structures in a selective manner. The detailedexample provided below focuses on using biotin- and streptavidin-coatedmicrospheres as the basis for the photonic crystals. Streptavidin is aprotein while biotin is a vitamin. These are well-known molecules whichbond with one another in a biological bond with a strength approachingthat of a covalent bond, but the spheres do not tend to bond tothemselves (aggregate). Other types of biological recognition such asprotein to protein, DNA to DNA, etc. will be described later. Thesecoated microspheres are readily available commercially, and are commonlyused in the process of fluorescence microscopy to indicate the presenceof other proteins.

[0031] In accordancewith certain embodiments of the present invention, alayer-by-layer approach is taken to fabrication of a photonic crystal.Many variations of techniques consistent with the current teachings arepossible without departing from the invention. One embodiment using theabove-mentioned biotin- and streptavidin coated spheres is illustratedin FIGS. 1-4 wherein a three layer photonic crystal is constructed usingsuch microspheres. In this exemplary embodiment, alternating layers areconstructed using biological bonds (i.e., protein bonds) to secure thelayers together and to a substrate. Turning first to FIG. 1, a substrate100 can be “templated” or “patterned” to capture microspheres of thedesired size in a desired arrangement. In this simple example, thetemplating is done by starting with a metal coated substrate (e.g., agold plated substrate) and etching away metal to form regularly spaceddisks of metal such as 104 using any suitable etching technique.

[0032] Suitable substrates can be devised of many types of materialsincluding, but not limited to, glass, quartz, silicon, germanium,gallium arsenide, photoresist, ceramic, epoxies, polymers, plastics, andmetals: The main consideration, from the perspective of actuallayer-by-layer fabrication of the photonic crystal, is that thesubstrate have a surface for bonding of the microspheres which is verysmooth in comparison with the size of the microspheres.

[0033] The disks 104 are sized to accommodate a single layer ofmicrospheres with a single microsphere centered on each disk 104. Thus,if 2.0 micron diameter microspheres are used, the disks would be spaced2.0 microns apart center to center. The templating technique used, ofcourse, should be matched to the resolution required to achieve theprecision needed to accommodate a particular size of microspheres. Thesubstrate can be of any suitable material such as previously described.

[0034] For this exemplary embodiment, the metal disks remaining from theetching are then treated to make them suitable for bonding with thedesired first layer of microspheres. In the present exemplaryembodiment, the first layer can be made of streptavidin-coatedmicrosphere beads (available from Bangs Laboratories, Inc., 9025Technology Drive, Fishers, Ind. 46038 and other commercial sources).Thus, in the present example, to permit bonding with thestreptavidin-coated microspheres the disks 104 are first plated withgold and then biotinylated using any known process for biotinylatinggold. In other embodiments, any mechanism for biotinylating a specifiedregion of the substrate can be used to create regions of preferentialbonding for the streptavidin-coated microspheres. Since streptavidinreadily forms a biological bond with biotin, but will not bond withitself, a first layer of microspheres can be applied to the substrate bysuspending the microspheres in a colloidal solution or slurry andbringing the solution or slurry into contact with the templatedsubstrate 100. This produces a single layer of streptavidinmicrospheres. Since streptavidin spheres do not readily bond tothemselves, this layer is referred to herein as “self-passivated”. Thatis, once the single layer is formed, other streptavidin spheres do nottend to bond to the single layer. (Other equivalent techniques couldhypothetically involve applying microspheres that do not explicitlyself-passivate, if excess microspheres can then be removed to leave asingle layer. Such equivalents to self passivated layers arecontemplated by the present invention.)

[0035] Many variables are possible in this stage, including but notlimited to, the temperature, pH, slurry concentration, agitation,composition of the liquid component of the slurry and contact time.These variables can be optimized by experimentation to achieve a desiredcoating of a single layer of microspheres such as illustrated in FIG. 2.In this figure, the streptavidin-coated microspheres 110 preferentiallysituate themselves in a single layer with the disks 104 and bond to thedisks with a very strong protein bond. That is, a layer of microsphereswhich is one microsphere deep is deposited on the substrate and bondstherewith.

[0036] Once a complete layer of the streptavidin-coated microspheres 110is bonded to the array of disks 104, the remaining streptavidin-coatedmicrospheres present in the slurry or solution have nowhere to bond andcan be readily rinsed away, for example with a water rinse (andpotentially reused). At this stage, a single layer photonic crystallinestructure 120 is formed and is precisely a single layer of microspheresin thickness above the substrate 100.

[0037] In orderto continue building a thicker photonic crystal, a secondlayer (again one microsphere deep) can be added. In this example, asecond layer of streptavidin-coated microspheres will not bond to thefirst layer of streptavidin-coated microspheres 110. A second layer,however, can be constructed using a complementary type of microspherethat will form a protein bond with the streptavidin-coated microspheres110 as illustrated by the two layer photonic crystal illustrated as 130in FIG. 3. In order to apply a second layer of microspheres, biotincoated microspherical beads 134 of the same size as thestreptavidin-coated microspheres 110 are used (also available from BangsLaboratories, Inc., 9025 Technology Drive, Fishers, Ind. 46038 and othercommercial sources). The biotin coated microspheres 134 are again placedin a suitable slurry and the single layer photonic crystal 120 isexposed to the slurry (again with variables such as the temperature, pH,slurry concentration, agitation, composition of the liquid component ofthe slurry and contact time optimized by experimentation to achieve adesired coating of a single layer of biotin coated microspheres 134bonded to the streptavidin-coated microspheres 110). Under suitableconditions, the second layer will form a strong biological bond with thefirst layer in a single layer mechanical arrangement as illustrated. Thepacking of the second layer is provided by the topographic features ofthe first layer of spheres (110), wherein multiple bonding surfaces(four surfaces in the example shown in FIG. 3) are available to thesecond layer at the points between spheres, providing the most stablearrangement. In addition to a larger number of bonding surfaces, thereduced amount of Brownian fluid forces on the microsphere once it islocated in a recessed area will assist in stabilizing the second layerin an arrangement consistent with that of the first layer. Thus thepacking of the second layer 134 is determined by the pattern provided bythe first layer 110, such that a variety of three-dimensional structurescan be obtained by modifications to the arrangement of microspheres inthe first layer. As with the first layer, the second layerself-passivates when no more spaces are available on the first layer ofstreptavidin-coated microspheres 110 for bonding by the biotin coatedmicrospheres 134. The remaining biotin coated microspheres in the slurrycan then be rinsed away. Thus, the second layer is formed to produce atwo layer photonic crystal structure 130.

[0038] A third layer can be fabricated using a similar technique to thatof the first layer. To form the third layer, streptavidin-coatedmicrospheres are again used since they will bond with the biotin coatedmicrospheres 134 of the second layer with a protein bond. Sincestreptavidin readily forms a protein bond with biotin, but will not bondwith itself, a third layer of microspheres can again be applied to thesubstrate by placing the streptavidin microspheres in a liquid solutionor slurry and bringing the slurry into contact with the two layerphotonic crystal 130. Again, many variables are possible in this step,including but not limited to, the temperature, pH, slurry concentration,agitation, composition of the liquid component of the slurry and contacttime. These variables can be optimized by experimentation to achieve adesired coating of a single layer of microspheres 140 (one microspheredeep) bonded to the layer of microspheres 134 such as illustrated inFIG. 4. In this figure, the streptavidin-coated microspheres 140preferentially situate themselves in a single layer with themicrospheres 134 and bond to the microspheres 134 with a very strongprotein bond. Once a complete layer of the streptavidin-coatedmicrospheres 140 is bonded to the layer of microspheres 134, theremaining streptavidin-coated microspheres present in the slurry havenowhere to bond and can be readily rinsed away and potentially reused.At this stage, a three layer photonic crystal 150 is formed.

[0039] This process can be repeated sequentially building layer afterlayer of alternating streptavidin-coated microspheres and biotin-coatedmicrospheres to form a desired sized and shaped crystalline structure.

[0040] Initial experiments have been conducted to fabricate a photoniccrystal using the above-described method. Results show that the twotypes of microspheres do in fact preferentially bond with one another,and that a two-layer structure can be deposited on a glass slide servingas an unpatterned (untemplated) substrate. In these experiments, thiswas accomplished by bonding streptavidin coated spheres to a glass slideto create an unpatterned irregular monolayer by placing a drop of watercontaining 1% solids (1 mg of microspheres per 100 mL of water) in achamber on a glass slide created by bordering a section of the slidewith tape and placing a cover over the section. The streptavidin spheresnaturally bond to glass to create an irregular monolayer ofstreptavidin. After flushing away excess streptavidin-coated sphereswith water, this monolayer was then exposed to a 1% solids solution(again 1 mg of solids per 100 milliliters of water) of biotin-coatedmicrospheres at room temperature for 30 seconds to five minutes. In thisexample, the substrate and thus the layers are not patterned, so theresult was a two-layer random arrangement of microspheres. Experimentalresults show that higher concentrations of the spheres in the solution,and longer waiting time for the solution to settle (without agitation)resulted in higher concentration of deposited and bonded spheres. Theexperiments suggest that use of a patterned or templated substrate asdescribed above would permit engineering of the mechanical structure inthe layer-by-layer fashion described above.

[0041] Selection of an appropriate substrate material depends upon thetype and size of microsphere and type of bonding to be used to assemblethe layers, as well as the patterning or templating to be used.

[0042] Patterning the substrate to control the first layer ofmicrospheres may present a technical challenge, depending on the size ofthe microspheres to be used, but substrate templating has beendemonstrated in the literature for colloidal assembly approaches, andtemplating the substrate with respect to the present method should notpresent any new challenges. The continuous improvements in the abilityto pattern substrates are expected to permit smaller and smallermicrospheres to be used. The particular approach used to pattern ortemplate the substrate depends, in part, on the type of chemistry usedfor bonding the microspheres together. It may also be possible toproduce multiple layers using the technique described without use of atemplated substrate, although templating is preferred.

[0043] Many methods could potentially be used to template the substrate.Generally, methods of templating the substrate that can potentially beused include chemical patterning, physical patterning, or a combinationof the two.

[0044] Physical patterning of the topography of the substrate can alsopotentially be used, for example, as illustrated in FIGS. 5 and 6. Oneapproach to physical patterning would use anisotropic etching ofperiodic inverted pyramids such as 204 and 206 into the substrate 212 atan upper surface 216 thereof. Using four sided pyramids, such as pyramid204 this would give the microspheres four bonding sites at each of theetched surfaces 220, 222, 224 and 226 in the pyramid rather than justone on a plane surface such as the upper surface 216, and it is expectedthat the spheres would preferentially settle and bond inside thepyramids such as 204 and 206. Inverted pyramidal shapes having three ormore sides can be used without departing from the present invention.Moreover, pyramids are only one shape that could be etched or otherwisemilled into a substrate to provide multiple bonding sites that sphereswould likely consider preferential to a single surface site. Othertechniques, such as isotropic etching; mechanical machining; molding;micro-contact printing; UV, deep UV, x-ray, electron beam, or ion beamlithography; ion beam milling; holographic patterning; two-photonpolymerization; or some other suitable technique can also equivalentlybe used, in appropriate combinations if necessary, to pattern a surfaceof the substrate.

[0045] Topographical patterning generally uses a substrate with uniformchemical composition, but this should not be considered limiting sincemany exceptions can be conceived. The topographical patterns providepreferential bonding sites for the beads based on physical topography.Examples are rounded or pyramidal recesses in the substrate surface. Therecesses give larger areas for bonding or additional surfaces forformation of bonds between the microspheres and the substrate.

[0046] Chemical selectivity can be used to template the substrate bycreating local regions of the substrate where the microspheres tend tobond with the substrate by use of a chemical with appropriate bondingproperties. The remainder of the substrate can be made of a differentmaterial. One example would be to use a background material which doesnot bond with streptavidin-coated beads, such as photoresist. Adjacentto the photoresist, regions are patterned which will form bonds withstreptavidin coated beads. Examples of such regions could includebiotinylated surfaces such as gold, or other surfaces such as glass,silicon, silicon dioxide, silicon nitride, etc. Chemical selectivity inthis context also includes bonding due to electrostatic or ionicattraction. Many other examples may occur to those skilled in the artupon consideration of this teaching.

[0047] A hybrid of chemical selectivity and topographical patterning canalso be used. In one example of such a hybrid approach,streptavidin-coated beads adhere to such surfaces as glass, silicon,silicon dioxide, silicon nitride, and gold. They do not adhere well tocertain types of photoresist. Patterning holes in the photoresist (viatraditional UV or electron beam lithography) results in a structure thatis chemically selective, and the height of the residual photoresistprovides a degree of physical templating as well. Either chemicalselectivity or physical templating of the substrate can potentially, butnot necessarily, be adaptable to include intentional defects into thecrystal structure if desired.

[0048] Thus, methods consistent with embodiments of the presentinvention can use forming a geometric pattern in the substrate materialto create preferential bonding regions on the substrate, formingthree-dimensional topography on a surface of the substrate to createpreferential bonding regions within the topography, forming invertedpyramid shaped recesses on a surface of the substrate material to createpreferential bonding regions within the inverted pyramids or chemicallytreating the substrate to create preferential bonding regions on thesubstrate. In addition, combinations of these techniques can be usedsuch as creating preferential bonding regions on the substrate by acombination of chemical and topographical patterning. Other techniquesmay occur to those skilled in the art upon consideration of the presentinvention.

[0049] The above process, as depicted in FIGS. 1-4, is outlined in flowchart 300 of FIG. 7 starting at 304. A suitable substrate is prepared orobtained at 308 so that spheres of a particular type (type A in thisexample) will bond to the substrate in a manner dictated by thepatterning or templating of the substrate. It is hypothesized that asuitable bond can result from many types of bonding and attractionphenomenon such as covalent bonding, electrostatic attraction, metallicbonding, hydrogen bonding (electrostatic attraction between anelectronegative atom and a hydrogen atom that is bonded covalently to asecond electronegative atom), Van der Waals forces,hydrophobic/hydrophilic attractions (hydrophobic attractions causenon-polar groups such as hydrocarbon chains to associate with oneanother in an aqueous environment), biological recognition such asprotein-protein/protein-ligand complexes (e.g., antigen-antibody), DNAor RNA hybridization or ligand-receptor (e.g., enzyme-substrate)(biological recognition generally results from a three dimensionalstructure that allows multiple weak forces between molecules), or somecombination of the above forces, or any other suitable bond. The spheresof type A are then brought into contact with the prepared surface of thesubstrate at 312 by, for example, immersing the substrate in a solutioncontaining the spheres or exposing the prepared surface to such asolution or slurry containing the microspheres. At this point, dependingupon the materials and type of bonding, agitation, heating or otheractions may be taken to enhance the speed or consistency of the bondingof the type A spheres to the substrate. Once a self-passivated (orotherwise equivalently self-limiting) single layer of spheres havebonded to the surface of the substrate, the excess type A spheres can beremoved, for example by rinsing at 316.

[0050] If multiple layers of spheres are desired at 320, the processproceeds to 324 where a second type of spheres (type B) is brought intocontact with the first layer of type A spheres bonded to the substrate.Type B spheres are spheres that bond to type A spheres but not tothemselves. Again, depending upon the materials and type of bonding,agitation, sonication, heating or other actions may be taken to enhancethe speed or consistency of the bonding of the type B spheres to thefirst layer of type A spheres. Once a self-passivated single layer oftype B spheres have bonded to the layer of type A spheres on thesubstrate, the excess type B spheres can be removed, for example byrinsing at 328. If only two layers are desired at 332, the process ishalted at 336. However, if additional layers are desired, one merelyrepeats 312 and 316 (with possible process adjustments to account forbonding between the two types of spheres rather than spheres tosubstrate).

[0051] This process can be repeated until a desired odd number of layersis reached at 320 or even number of layers is reached at 332, at whichpoint the process can be halted at 336. Those skilled in the art willappreciate that the present exemplary embodiment uses two differentcomplementary types of spheres, but this should not be consideredlimiting since the process can readily be expanded to as many types ofspheres as desired. Moreover, the process can be used to build a desiredstructure and at a desired position insert a different type of spherelayer to achieve controlled layering of a third type of material at aparticular location. Many variations will occur to those skilled in theart upon consideration of the examples provided herein without departingfrom the present invention.

[0052] In addition to the process described above, a variation of theprocess can be used (with or without the process above) to fabricatemulti-layer photonic crystalline structures. FIG. 8 is a flow chart 400describing one such variation of this process starting at 402. At 406 asuitable substrate is prepared or obtained so that spheres of aparticular type will bond to the substrate in a manner dictated by thepatterning or templating of the substrate as before. It is againhypothesized that a suitable bond can result from covalent bonding,electrostatic attraction, metallic bonding, hydrogen bonding, Van derWaals forces, hydrophobic/hydrophilic attractions, biologicalrecognition such as protein-protein/protein-ligand complexes (e.g.,antigen-antibody), DNA or RNA hybridization or ligand-receptor (e.g.,enzyme-substrate), or some combination of the above forces, or any othersuitable bond. The spheres are then brought into contact with theprepared surface of the substrate at 410 by, for example, immersing thesubstrate in a solution containing the spheres or exposing the preparedsurface to such a solution or slurry containing the microspheres. Atthis point, depending upon the materials and type of bonding, agitation,sonication, heating or other actions may be taken to enhance the speedor consistency of the bonding of the spheres to the substrate. Once aself-passivated or otherwise self-limiting single layer of spheres havebonded to the surface of the substrate, the excess spheres can beremoved, for example by rinsing at 414. Alternatively, the substrate isexposed to a layer of microspheres that weakly bond to it since a weakbond may be advantageous ordering. After deposition, the excesss pheresare gently removed and then the bond of the microspheres is strengthenedor activated by addition of additive chemicals such as glutaraldehyde,by change in pH, by UV or other radiation exposure or any othermechanism.

[0053] If additional layers are desired at 418, the layer of spheresbonded to the substrate are modified to permit bonding, either to thesame kind of sphere or to another kind of sphere at 422. The process of410, 414, 418 and 422 are then repeated until a crystal of desired sizeis achieved and the last layer is in place. The process is then haltedat 426.

[0054] The process of FIG. 8 could be carried out, for example, usingcovalent bonding. Microspheres coated with phosphonate terminal groupshave a 2-charge, and would exhibit self-passivation. When introduced toa solution containing Cr⁴⁺ ions, these ions bond to the phosphonategroups, leaving them with a +2 charge. Subsequent exposure tophosphonated microspheres would add an additional layer to thecrystalline structure. Thus, if phosphonated spheres are used in process400, the spheres can be modified as in 422 to permit them to bond to oneanother by introduction of a solution containing Cr⁴⁺ ions. Additionallayers of phosphonated microspheres can then be built up to produce aphotonic crystal made of a single type of sphere.

[0055] The process of FIG. 8 could also be carried out, for example,using biological recognition. One example of this type of process woulduse microspheres coated with biotin, and modified with streptavidin. At410 a self-passivated layer of biotin-coated spheres is formed in asimilar manner as described previously. Excess spheres are then removedby, for example, rinsing at 414. At 422, the spheres are exposed to asolution containing streptavidin, which bonds to the biotin-coatedsurface of the spheres. Each streptavidin protein contains four sitesfor bonding, and so three of those sites are still available forsubsequent bonding with biotin-coated spheres after the initial bond.Additional layers of biotinylated microspheres can then be built up toproduce a photonic crystal made up of a single type of sphere.

[0056] Note that the processes in FIG. 7 and FIG. 8 refer only to thebond types, so that other attributes of the microspheres, such as size,shape, and/or chemical composition, may vary within or between thelayers. In addition, one could perform additional processing steps, forexample introduction of defects within a layer, before proceeding to addsubsequent layers to the crystal.

[0057] As previously described, it is believed that many types of bondscan be utilized (singly or in combination) to fabricate crystalstructures consistent with certain embodiments of the present invention.Covalent coupling is a bonding technique that might be used toselectively deposit monolayers of spheres. Covalent coupling has theadvantage of providing the largest bond strength of the techniquesmentioned. In addition to the phosphonate/Cr⁴⁺ linkage mentionedpreviously, numerous covalent coupling chemistries are well known. Theyare often used to covalently couple carboxyl- or amino-modifiedmicrospheres to molecules such as proteins which have terminal carboxyland amino groups. Carboxyl- and amino-modified microspheres areavailable commercially from Bangs Labs, Inc (9025 Technology Drive,Fishers, Ind. 46038-2886) and other vendors. Several approaches forachieving covalent coupling between layers of microspheres in a photoniccrystal are possible. Examples include linkages betweencarboxyl-modified microspheres and ligands with available amines using awater soluble carbodiimide. One skilled in the art will appreciate that,several approaches can be taken using this type of chemistry. Oneexample would be to saturate a batch of carboxyl-modified microsphereswith carbodiimide, and use this as microsphere type “A” in the processflow shown in FIG. 7. Then amino-modified microspheres could be used asmicrosphere type “B”.

[0058] Another example of covalent coupling between microspheres usingthe process flow in FIG. 8 involves linkages between amino-modifiedmicrospheres using an amine-reactive homobifunctional cross-linker, suchas glutaraldehyde. The glutaraldehyde cross-linker bonds to amine groupson both ends of the molecule. Several approaches in applying this to theformation of photonic crystals are possible. For example, afterdepositing a layer of amino-modified microspheres, the layer could thenbe activated using a cross-linker, according to the process flow in FIG.8. A second approach would be to saturate a batch of amino-modifiedmicrospheres using the crosslinker and use this batch as type “A”microspheres, and then use the amino-modified microspheres without thecrosslinker as type “B” microspheres, and follow the process flow inFIG. 7.

[0059] Numerous other examples of covalent coupling reactions aredescribed in the literature involving, for example, hydroxyl, hydrazide,amide, chloromethyl, aldehyde, epoxy, and tosyl end groups, all of whichare available commercially as functional groups on the surface ofmicrospheres (e.g., from Bang Labs and other sources). Another exampleutilizes techniques involving the creation of self-assembled monolayers(SAMs). Such techniques involve exposing a type of molecule, usually insolution, to a surface which bonds to one endgroup of the molecule. Theother endgroup may be functionalized for a variety of applications. Manyother covalent coupling chemistries will occur to those skilled in theart upon consideration of the present teaching.

[0060] As previously described, many types of bonds can be utilized tofabricate crystal structures consistent with embodiments of the presentinvention. Electrostatic charge is another bonding technique that mightbe used to selectively deposit monolayers of spheres. Electrostaticbonds occur between ionized groups of opposite charge (e.g., carboxyl(—COOH and amino (—NH₂)). It is preferred that spheres used in anelectrostatic bonding embodiment have an electrostatic charge that islocalized to the surface of the sphere. If the electrostatic charge iscentralized, as the charge is shifted away from the center of thesphere, the spheres might cluster and lose the self-passivation qualitydesired for the present invention. Several approaches are possible forcreating a charge localized to the surface of the sphere. Silica spheresnaturally have a negative surface charge due to the Si—H terminations atthe surface. Therefore the charge is inherently distributed on thesphere at the surface. Using this negative charge, it is relatively easyto coat the spheres leaving a positive charge, again distributed on thesurface. Other examples of microspheres with alternative surfacechemistries that exhibit electrostatic charges and are commerciallyavailable are polymeric microspheres with carboxyl surface groups, whichare negative, and amine surface groups, which are positive.Alternatively, the electrostatic charge could be generated in severalways such as ion implantation or plasma processing. Other surfacechemistries that exhibit electrostatic charge are consistent withcertain embodiments of the present invention.

[0061] Another example consistent with certain embodiments of thepresent invention, takes advantage of a common technique in biomaterialsresearch known as protein, RNA, DNA, or more generally, biologicalrecognition. A protein, RNA or DNA strand is bound to a surface, andwhen exposed to the antigen complementary RNA or DNA strand, or RNA orDNA binding protein the two compatible materials are strongly bound toeach other. This is normally detected by techniques such as fluorescenceor surface plasmon resonance spectroscopy. The same approach could beapplied to spheres, with alternating layers of spheres having surfacescomposed of protein and antigen, respectively for example. An advantageof this approach is the high selectivity provided by the proteins orDNA. A potential advantage of DNA or RNA coupling between microspheresis that the length of the strands can be selected over a wide range oflengths. This could allow tuning of the lattice parameters of thecrystal independent of sphere size. Also, the strands are elasticcompared to most molecules, which could allow easier tuning of thephotonic bandgap by mechanical stretching.

[0062] Using biological recognition, as described previously, a photoniccrystal could be formed by using alternating layers of biotin- andstreptavidin-coated spheres, respectively. Alternatively, a photoniccrystal could be formed using only one type of microsphere, in this casebiotin-coated microspheres, such that after each layer is deposited, themicrospheres are treated with a complemetary molecule, in this casestreptavidin, such that they are activated to bond with a subsequentlayer of microspheres.

[0063] Another use of biological recognition could involve one layerfunctionalized with a single strand DNA or RNA and the following layer acomplementary (antisense) oligonucleotide strand. Similarly, alternatinglayers providing antibody/antigen binding could be used, e.g. withprotein A and IgA, or protein G and IgG. Enzyme and substrate binding isnormally temporary, but this binding could be used between alternatinglayers by locking the molecules in an intermediate bound state by usingappropriate buffer conditions.

[0064] Another method for obtaining a self-passivated layer that appearsparticularly promising is the use of electrostatic charges in chargedpolymers such as polyelectrolytes. Polyelectrolytes are polymers withionizable groups on each monomer repeat unit. The deposition of a thinfilm layer can be achieved by adsorbing a polyelectrolyte of one chargefrom solution onto an immersed substrate, which may be planar, patternedor microspheres. Adsorption takes place with a simple dipping, but alsowith spraying, spin-coating and polymer stamping. Adhesive propertiesand tuning of the film characteristics can be achieved by multilayerdeposition. A substrate with a first polyelectrolyte film is capable ofadsorbing a second polyelectrolyte film of the opposite charge. Theprocess can be repeated alternating between the two polyelectrolytesolutions of opposite charge. This deposition is called layer-by-layerbut this terminology should not be confused with the layer-by-layermicrosphere deposition invented here. The polyelectrolyte multilayerproperties such as thickness and surface charge density can be tunedusing, among others, the number of layers, ionic strength, pH andsurfactants.

[0065] Polyelectrolytes can be of positive and negative charge and arecalled “weak” if the degree of ionization is pH dependent and “strong”otherwise. An example of a negative strong polyelectrolyte isPoly(sodium 4 styrenesulfonate) (PSS for short) [from Sigma-Aldrich,product number 527483], a positive strong polyelectrolyte is PDDA(Poly(diallyldimethylammonium chloride) [from Sigma-Aldrich, productnumber 409014], a positive weak polyelectrolyte is PAH Poly(allylamine)hydrochloride Aldrich 479144, ca. 65000]

[0066] The polyelectrolyte multilayers can be used as bonds to createthe layer-by-layer colloid crystals described herein. The bond between afirst layer of beads and the substrate can be fabricated by coating thesubstrate with a single- or multi-layer ending with a positive surfacecharge and using negatively charge microspheres such as carboxylatedpolystyrene microspheres, or by using microspheres with apolyelectrolyte coating ending with a negative surface charge. Bondsbetween microspheres can be achieved by using two kinds of microspheres,for example a 3 layer film of PDDA/PSS/PDDA creates positive surfacecharge on polystyrene microspheres that electrostatically bind tocarboxylated microspheres.

[0067] In another embodiment, the colloid crystal can be formed byalternating layers of spheres coated with an even-numberedpolyelectrolyte multilayer and sphere coated with an odd-numberedpolyelectrolyte multilayer.

[0068] In another embodiment, the microspheres deposited as first layerare coated, after their adhesion on the substrate, with polyelectrolytefilm of opposite charge and become in this way an adhesive substrate fora second layer of the same kind of spheres. Other variations will occurto those skilled in the art upon consideration of the above teachings.

[0069] Many of the bond types described previously are typicallyproduced in solution, but many, such as phosphonate/chromium bonds, canbe dried after bonding occurs. This means that no matrix, liquid orsolid, is necessary to bind the crystal together, although one could beused if desired or if otherwise beneficial. Removing the matrix materialmay significantly improve the index of refraction contrast between thespheres and surrounding medium (air), which is a major factor in theoptical behavior of the crystalline structure. A table of commonmaterials used as microspheres and/or matrix is given below. Removal ofthe matrix also improves the ability to introduce arbitrary liquid orvapor phase environments to the crystal so that it can more easily beused as a sensor. Removal of the liquid matrix would also allowinfiltration and densification of the volume between microspheres withsemiconductors or metals, which has been demonstrated using thetraditional colloidal approach. Subsequent removal of the microspheresis then possible. Material Index of refraction Air 1.00 Water 1.33Silica 1.46 Polystyrene 1.59

[0070] There are several benefits to use of the methods described above.For example, with proper experimental conditions, defect levels areexpected to be much lower than for traditional colloidal techniques.Also, traditional colloidal techniques are limited to sphere sizes inthe 800-1000 nm range or smaller, because sedimentation effects becomeimportant for larger spheres. Because the current approach builds thecrystal a single layer at a time, sedimentation is not a concern. Thisshould allow larger spheres to be used so that crystals with opticalfeatures in the mid- and far-infrared can be fabricated. As mentionedpreviously, substrate templating and proper sphere size selection canallow more complex crystal structures than were previously possible. Forexample, non-close-packed structures such as body-centered cubic,tetragonal, and monoclinic structures can be designed and fabricated.Also, the crystals can be integrated into optical systems on a waferscale, and batch processing should make the process very cost-efficient.

[0071] Thus, in accordance with certain embodiments, the currentinvention provides several new techniques for the fabrication ofphotonic crystals composed of small particles, such as spheres. Thislayer-by-layer approach as described herein allows one to tailor theproperties of the crystalline structure in ways not previously possible,which could not only speed the development of commercially viablecrystals, but allow the design of structures with new functionality.

[0072] While the invention has been described in conjunction withspecific embodiments, it is evident that many alternatives,modifications, permutations and variations will become apparent to thoseskilled in the art in light of the foregoing description. Accordingly,it is intended that the present invention embrace all such alternatives,permutations, modifications and variations as fall within the scope ofthe appended claims.

What is claimed is:
 1. A method of fabricating a photonic crystal,comprising: providing a substrate; exposing the substrate to a pluralityof first microspheres made of a first material, the first material beingof a type that will bond to the substrate and form a self-passivatedlayer of first microspheres to produce a first layer; and exposing thefirst layer to a plurality of second microspheres made of a secondmaterial, the second material being of a type that will bond to thefirst layer and form a self-passivated second layer of secondmicrospheres.
 2. The method according to claim 1, further comprising:exposing the second layer to a plurality of the first microspheres madeof a the first material, the first material being of a type that willbond to the second layer and form a self-passivated layer of firstmicrospheres.
 3. The method according to claim 2, further comprising:repeatedly exposing a most recently formed layer to microspheres to aplurality of microspheres that will bond to the most recently formedlayer and self-passivate to fabricate a multiple layer photonic crystal.4. The method according to claim 1, wherein the first microspherescomprise streptavidin-coated microspheres and the second microspherescomprise biotin coated microspheres.
 5. The method according to claim 4,wherein the substrate has biotinylated regions on a surface of thesubstrate.
 6. The method according to claim 1, wherein the firstmicrospheres comprise biotin-coated microspheres and the secondmicrospheres comprise streptavidin-coated microspheres.
 7. The methodaccording to claim 1, wherein the bond comprises at least one ofcovalent bonding, electrostatic attraction, metallic bonding, hydrogenbonding, Van der Waals forces, hydrophobic/hydrophilic attractions andbiological recognition.
 8. The method according to claim 1, wherein oneof the first and second microspheres have DNA strands on a surfacethereof, and wherein the other of the first and second microspheres haveat least one of complimentary DNA strands, complimentary RNA strands,oligonucleotides and DNA binding proteins on a surface thereof.
 9. Themethod according to claim 1, wherein one of the first and secondmicrospheres have RNA strands on a surface thereof, and wherein theother of the first and second microspheres have at least one ofcomplimentary DNA strands, complimentary RNA strands, oligonucleotidesand DNA binding proteins on a surface thereof.
 10. The method accordingto claim 1, wherein one of the first and second microspheres have aprotein situated on a surface thereof, and wherein the other of thefirst and second microspheres have at least one of an antigen and aligand that bonds to the protein on a surface thereof.
 11. The methodaccording to claim 1, wherein the first microspheres have a firstmolecule with a first endgroup on a surface thereof, and wherein thesecond microspheres have a second molecule with a second endgroup on asurface thereof, wherein the first and second molecules bond to eachother, but not to themselves, by formation of one of a covalent, ionic,metallic, hydrogen and Van der Waals bond.
 12. The method according toclaim 1, wherein one of the first and second microspheres have a bulkelectrostatic charge or a surface electrostatic charge of a first chargestate, and wherein the other of the first and second microspheres have asecond bulk electrostatic charge or surface electrostatic charge with asecond charge state which is opposite and attractive to the first chargestate, wherein the first and second microspheres bond to each other byformation of ionic/electrostatic bonds, but do not bond to themselves.13. The method according to claim 1, further comprising processing thefirst layer to form a surface that will bond to the second microspheresprior to exposing the first layer to the plurality of microspheres. 14.The method according to claim 1, wherein the substrate has a surfacecharge of a first polarity and wherein the first microspheres have acharge of a second polarity, and wherein the second microspheres have acharge of the first polarity.
 15. The method according to claim 1,wherein the first and second microspheres are coated with first andsecond polyelectrolyte layers, wherein the first and secondpolyelectrolyte layers have opposite charge.
 16. A method of fabricatinga photonic crystal, comprising: a) providing a substrate; b) exposingthe substrate to a plurality of first microspheres made of a firstmaterial, the first material being of a type that will bond to thesubstrate and form a self-passivated layer of first microspheres toproduce a layer of microspheres; c) modifying the first layer ofmicrospheres to permit the first layer of microspheres to bond withother microspheres to thereby produce a bondable layer; and d) exposingthe bondable layer to a plurality of second microspheres to form asecond layer of microspheres.
 17. The method according to claim 16,wherein the plurality of second microspheres are made of the firstmaterial.
 18. The method according to claim 16, wherein the plurality ofsecond microspheres are made of a second material.
 19. The methodaccording to claim 16, further comprising: modifying the second layer ofmicrospheres to permit the second layer of microspheres to bond withother microspheres and thereby produce a second bondable layer; exposingthe second bondable layer to a plurality of microspheres to form a thirdself-passivated layer of microspheres to produce a three layer photoniccrystal.
 20. The method according to claim 16, further comprisingrepeating c) and d) a plurality of times to achieve a desired number oflayers of a photonic crystal.
 21. The method according to claim 16,wherein the bond comprises at least one of covalent bonding,electrostatic attraction, metallic bonding, hydrogen bonding, Van derWaals forces, hydrophobic/hydrophilic attractions and biologicalrecognition.
 22. The method according to claim 16, further comprisingactivating the bond of the microspheres by at least one of thefollowing: addition of additive chemicals such as glutaraldehyde, bychange in pH, and by exposure to radiation.
 23. The method according toclaim 16, wherein the first microspheres have a first charge, andwherein the modifying comprises coating the first microspheres with apolyelectrolyte film having charge opposite the first charge.
 24. Themethod according to claim 23, wherein the second microspheres also havethe first charge.
 25. A photonic crystal structure, comprising: asubstrate processed to bond preferentially to a first material inselected areas; a first layer of first microspheres, the first layerbeing one microsphere deep, the first microspheres comprising the firstmaterial and bonded to the selected areas of the substrate; and a secondlayer of second microspheres one microsphere deep and bonded to thefirst layer of microspheres.
 26. The apparatus according to claim 25,wherein one of the first and second microspheres comprisestreptavidin-coated microspheres and the other of the first and secondmicrospheres comprise biotin coated microspheres.
 27. The apparatusaccording to claim 25, wherein one of the first and second microsphereshave RNA strands on a surface thereof, and wherein the other of thefirst and second microspheres have at least one of complimentary DNAstrands, complimentary RNA strands, oligonucleotides and RNA bindingproteins on a surface thereof.
 28. The apparatus according to claim 25,wherein the one of the first and second microspheres have DNA strands ona surface thereof, and wherein the other of the first and secondmicrospheres have at least one of complimentary DNA strands,complimentary RNA strands, oligonucleotides and DNA binding proteins ona surface thereof.
 29. The apparatus according to claim 25, wherein oneof the first and second microspheres have a protein situated on asurface thereof, and wherein the other of the first and secondmicrospheres have at least one of an antigen and a ligand that bonds tothe protein on a surface thereof.
 30. The apparatus according to claim25, wherein first microspheres have a first molecule on a surfacethereof, and wherein the second microspheres have a second molecule on asurface thereof, wherein the first and second molecules bond to eachother but not to themselves.
 31. The apparatus according to claim 25,wherein the first microspheres have a first bulk or surfaceelectrostatic charge, and wherein the second microspheres have a secondbulk or surface electrostatic charge which is opposite and attractive tothe first electrostatic charge, wherein the first and secondmicrospheres bond to each other but not to themselves.
 32. The apparatusaccording to claim 25, wherein the bond comprises at least one ofcovalent bonding, electrostatic attraction, metallic bonding, hydrogenbonding, Van der Waals forces, hydrophobic/hydrophilic attractions andbiological recognition.
 33. The apparatus according to claim 25, whereinthe second microspheres are comprised of a second material.
 34. Theapparatus according to claim 25, wherein the second microspheres arecomprised of the first material.
 35. The apparatus according to claim25, wherein the substrate has a surface charge of a first polarity andwherein the first microspheres have a charge of a second polarity, andwherein the second microspheres have a charge of the first polarity. 36.The apparatus according to claim 25, wherein the first and secondmicrospheres are coated with first and second polyelectrolyte layers,wherein the first and second polyelectrolyte layers have oppositecharge.
 37. A method of fabricating a photonic crystal, comprising:providing a substrate; bonding a single layer of microspheres onemicrosphere deep to the substrate to form a first layer; and bonding asingle layer of microspheres one microsphere deep to the first layer toform a second layer.
 38. The method according to claim 37, furthercomprising repeatedly bonding a layer of microspheres one microspheredeep to a most recently formed layer to produce a multiple layerphotonic crystal.
 39. The method according to claim 37, wherein the bondcomprises at least one of covalent bonding, electrostatic attraction,metallic bonding, hydrogen bonding, Van der Waals forces,hydrophobic/hydrophilic attractions and biological recognition.
 40. Themethod according to claim 38, further comprising modifying the mostrecently formed layer to cause the layer to bond with a next layer ofmicrospheres.
 41. The method according to claim 37, wherein alternatinglayers of the multiple layer photonic crystal are comprised ofmicrospheres of differing types.
 42. The method according to claim 37,wherein the substrate has a surface charge of a first polarity andwherein the first microspheres have a charge of a second polarity, andwherein the second microspheres have a charge of the first polarity. 43.The method according to claim 37, wherein the first and secondmicrospheres are coated with first and second polyelectrolyte layers,wherein the first and second polyelectrolyte layers have oppositecharge.
 44. A method of fabricating a photonic crystal, comprising:providing a templated substrate having a first charge; and exposing thetemplated substrate to a plurality of first microspheres having apolyelectrolyte coating carrying a second charge, the second chargebeing opposite the first charge so that the plurality of firstmicrospheres will bond to the templated substrate and form aself-passivated layer of first microspheres to produce a first layer.45. The method according to claim 44, further comprising: exposing thefirst layer to a plurality of second microspheres having apolyelectrolyte coating carrying the second charge in order to bond tothe first layer and form a self-passivated second layer of secondmicrospheres.
 46. The method according to claim 45, further comprising:exposing the second layer to a plurality of the first microsphereshaving a polyelectrolyte coating carrying the first charge in order tobond to the second layer and form a self-passivated layer of firstmicrospheres.
 47. The method according to claim 46, further comprising:repeatedly exposing a most recently formed layer to microspheres to aplurality of microspheres coated with a charged polyelectrolyte coatingthat will bond to the most recently formed layer and self-passivate tofabricate a multiple layer photonic crystal.
 48. The method according toclaim 47, wherein a last layer comprises carboxylated microspheres. 49.The method according to claim 45, wherein the first and secondmicrospheres are coated with one of Poly(sodium 4 styrenesulfonate) andPoly(diallyldimethylammonium chloride).
 50. A method of fabricating aphotonic crystal, comprising: a) providing a templated substrate; b)exposing the templated substrate to a plurality of first microspheresmade of a first material, the first material being of a type that willbond to the templated substrate and form a self-passivated layer offirst microspheres to produce a layer of microspheres; c) modifying thefirst layer of microspheres to permit the first layer of microspheres tobond with other microspheres to thereby produce a bondable layer bycoating the first microspheres with a polyelectrolyte film having afirst charge; and d) exposing the bondable layer to a plurality ofsecond microspheres having charge opposite the first charge to form asecond layer of microspheres.
 51. The method according to claim 50,further comprising: modifying the second layer of microspheres to permitthe second layer of microspheres to bond with other microspheres andthereby produce a second bondable layer by coating the second layer witha polyelectrolyte film; exposing the second bondable layer to aplurality of microspheres to form a third self-passivated layer ofmicrospheres to produce a three layer photonic crystal.
 52. The methodaccording to claim 51, further comprising repeating c) and d) aplurality of times to achieve a desired number of layers of a photoniccrystal.
 53. The method according to claim 50, wherein the first andsecond microspheres are coated with one of Poly(sodium 4styrenesulfonate) and Poly(diallyldimethylammonium chloride).
 54. Aphotonic crystal structure, comprising: a templated substrate processedto bond preferentially to a first material in selected areas; a firstlayer of first microspheres, the first layer being one microsphere deep,the first microspheres comprising the first material and bonded to theselected areas of the templated substrate; and a charged polymer coatingon the first microspheres.
 55. The apparatus according to claim 54,further comprising a second layer of second microspheres one microspheredeep and bonded to the first layer of microspheres, the secondmicrospheres having a charge that bonds to the charged polymer coating.56. The apparatus according to claim 54, wherein the charged polymercomprises a polyelectrolyte.
 57. The method according to claim 56,wherein the charged polymer comprises one of Poly(sodium 4styrenesulfonate) and Poly(diallyldimethylammonium chloride).
 58. Amethod of fabricating a photonic crystal, comprising: providing atemplated substrate; bonding a single layer of charged polymer coatedmicrospheres one microsphere deep to the templated substrate to form afirst layer; and bonding a single layer of charged polymer coatedmicrospheres one microsphere deep to the first layer to form a secondlayer.
 59. The method according to claim 58, further comprisingrepeatedly bonding a layer of charged polymer coated microspheres onemicrosphere deep to a most recently formed layer to produce a multiplelayer photonic crystal.
 60. The apparatus according to claim 58, whereinthe charged polymer comprises a polyelectrolyte.
 61. The methodaccording to claim 60, wherein the charged polymers are selected fromPoly(sodium 4 styrenesulfonate) and Poly(diallyldimethylammoniumchloride).
 62. A method of fabricating a photonic crystal, comprising:bonding a single layer of charged polymer coated microspheres onemicrosphere deep to a substrate to form a first layer; and bonding asingle layer of charged polymer coated microspheres one microsphere deepto the first layer to form a second layer.
 63. The method according toclaim 62, further comprising repeatedly bonding a layer of chargedpolymer coated microspheres one microsphere deep to a most recentlyformed layer to produce a multiple layer photonic crystal.
 64. Theapparatus according to claim 62, wherein the charged polymer comprises apolyelectrolyte.
 65. The method according to claim 64, wherein thecharged polymers are selected from Poly(sodium 4 styrenesulfonate) andPoly(diallyldimethylammonium chloride).